dodge ram 2500 diesel ac/heating problems

I have a 1999 Dodge Diesel 2500.

The AC/Heating system isn't working properly. If i turn up the heat it makes the temperatrue really hot. Once the heat is on it won't turn off. The fan works fine. I end up turning the whole system off and just rolling down the window but i would like to be able to use the AC.
I already tried to replace the whole control unit. But that didn't do anything.

Any suggestions?

Thanks